1. Field of the Invention
The present invention relates generally to the field of "uninterruptable" power supplies and, more specifically, to an uninterruptable power supply which includes an apparatus for controlling the application of a primary power source and a battery power to a load.
2. Discussion of the Prior Art
Many types of electronic equipment are powered by batteries to achieve desired functionality. In smaller equipment, such as laptop computers, it is often desirable to use a battery power supply for portability. Larger equipment, such as a "minicomputer" or mainframe computer, may rely on a battery power source as a back-up which is switched on-line during a failure in a primary power source (i.e., the local electric utility). Such "uninterruptable" power supplies are thus intended to maintain a continuous supply of power in order to prevent or reduce down time of costly, critical equipment and also prevent loss of data during interruptions of utility supplied power.
A conventional uninterruptable power supply is connected to an AC primary power source, sometimes referred to herein as the "line." If the load served by the power supply requires DC as well as AC voltage, then the power supply includes one or more rectifiers to produce a DC voltage. The power supply also typically includes an array of batteries connected together to form a back-up power source capable of delivering sufficient current to power the load for a short time period. Finally, one or more power conversion stages are usually provided to convert the AC line voltage or rectified line voltage (or DC battery voltage) to appropriate levels for the load. Such conversion stages may also perform filtering or regulation functions. One type of conversion stage, known as an inverter, may be used to convert a rectified line voltage or DC battery voltage to a regulated and filtered AC voltage which is suitable for application to the load.
Conventional uninterruptable power supplies exhibit several disadvantages, however. One major disadvantage is that a separate battery power conversion stage is often required to convert the DC voltage produced by the battery power source to appropriate AC or DC levels for the load. This battery power conversion stage is costly to implement, yet little used, since it is needed only during presumably short, infrequent failures of the primary power source. Moreover, the presence of the battery power conversion stage adds to the packaging volume of the power supply and may interfere with or prevent the attainment of other design objectives such as "modularity" (i.e., the ability to construct the power supply from discrete units having common interfaces).
A conventional alternative arrangement to providing a battery power conversion stage is to connect the battery power source directly to a DC bus, which in turn is connected to the power conversion stages which normally receive the rectified line voltage. The major disadvantage of this arrangement is that the inputs of the power conversion stages directly sourced by the batteries must be designed to accommodate input voltages which typically vary over large dynamic ranges. This is due to the fact that the magnitude of the voltage produced by the battery power source varies significantly as the batteries discharge. Again, such power conversion stages tend to be more expensive to implement than those with a more limited input range and also tend to adversely affect other design objectives.